Process for Producing Metal Flakes

Abstract

The present invention provides a jetting process for the production of flakes with uniform size distribution to be used in pigments comprising the steps of ejecting molten metal from a jet head and collecting droplets of metal on a solid collecting substrate or collecting droplets of metal in or on a collecting substrate.

Description

[0001]The present invention relates to a jetting process. The present invention further relates to flattened metal particulates, processes for their production and their use, especially as functional fillers and pigments.[0002]The term atomised metal powders is used in the industry to imply somewhat spherical particulates. They are thereby distinguished from flattened metal particulates, especially those flattened metal particulates of the metal pigment industry, which are generally referred to as flakes.[0003]Amongst metal flakes, aluminium and gold bronze (an alloy of copper and zinc) are the most widely manufactured, but copper, gold, iron, nickel, silver, stainless steel, tin, and zinc flakes are all commercially available. Applications of aluminium and gold bronze pigments are mainly for silver or gold coloration respectively of paints, inks, powder coatings and plastics. These and copper, gold, iron, nickel, silver, stainless steel, tin, and zinc metal flakes may also have fun...

AI Technical Summary

Benefits of technology

[0004]The preparation of metal flake pigments from conventionally atomised metal powders is well documented in the patent literature. Such pigments may be prepared in the complete absence of solvent by a dry ball milling process, but this can be hazardous in the case of reactive metals such as aluminium, due to the contaminating and / or explosive properties of the dry flake products. For such metals, dry milling has been largely superseded by wet ball milling processes in which metal powder is milled with an organic liquid such as mineral spirits and a small amount of a lubricant. The cascading action of grinding media within the ball mill causes the substantially spherical metal powder to be flattened out into flakes having aspect ratios (the ratio of the largest dimension to the smallest) of from about 15:1 to around 150:1 or even up to 250:1 and beyond. The function of the lubricant is to prevent the cold welding of adjacent flakes that can occur under the impact of grinding media.

[0010]Narrow particle size distribution is a highly desirable characteristic of metal flake pigments. Excessively fine flakes, such as those having a diameter of up to about 2 μm, have increased opacity or hiding power, but are substantially less brilliant than larger flakes. Coarser flakes, for example those with a diameter over 30 μm, may exhibit an undesirable degree of sparkle in modern coatings, in which a smooth visual effect is desired. Large flakes have substantially reduced opacity and can also cause problems in application. For example, when used in inks, they may block the cells of gravure printing presses.

Problems solved by technology

Such pigments may be prepared in the complete absence of solvent by a dry ball milling process, but this can be hazardous in the case of reactive metals such as aluminium, due to the contaminating and / or explosive properties of the dry flake products.

Firstly, it is technically difficult to produce high yields of metal powders with median particle diameters less than about 10 μm.

The twin limitations of fine powder production and wide particle size distribution have a consequential limiting effect on the characteristics of metal flakes prepared from atomised powders.

Coarser flakes, for example those with a diameter over 30 μm, may exhibit an undesirable degree of sparkle in modern coatings, in which a smooth visual effect is desired.

Large flakes have substantially reduced opacity and can also cause problems in application.

For example, when used in inks, they may block the cells of gravure printing presses.

Such a size segregation process adds to the cost of the desired product, as the removed material may have either no, or limited commercial use.

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